Method for preparing monolithic hydrated aluminas and composite materials

ABSTRACT

The subject of the present invention is a method of preparation of a monolithic hydrated alumina by the oxidation of aluminium or an aluminium alloy in the presence of a mercury amalgam that contains at least one noble metal, such as silver. This hydrated alumina serves inter alia as base product in methods of preparation of amorphous or crystalline aluminas, or of aluminates, which themselves serve as base products for methods of preparation of composite materials based on oxides, on metals, on carbon products and/or on polymers. Application of the said products obtained by the said methods in many fields, such as catalysis, thermal and acoustic insulation, magnetism, waste storage, and preparation of radioelement transmutation targets.

TECHNICAL FIELD

The present invention relates to a method of preparation of an ultraporous hydrated alumina in the form of monoliths, the said aluminaserving inter alia as a base product for the preparation of monolithic,amorphous or crystalline, anhydrous aluminas, monolithic aluminates andmonolithic composites.

PRIOR ART

The general field of the invention is therefore that of the preparationof aluminas.

In the prior art there are many methods of preparing aluminas byoxidation of aluminium in air.

Thus, in 1908, H. Wislicenus, in reference [1]: Kolloid Z., 2, 1908, 11,described the use of pure mercury deposited on a part made of aluminiumor aluminium alloy, so as to obtain alumina in the form of filaments orpowders. However, this method does not allow alumina to be obtained inmonolithic form, that is to say in the form of porous blocks.

More recently, J. Markel et al., in reference [2]: Journal ofNon-Crystalline Solids, 180 (1994), 32 described a process formanufacturing hydrated alumina monoliths, comprising a step ofdepositing mercury on an aluminium surface, the said mercury beingobtained by the reduction of mercuric ions in a nitric acid solution.This solution firstly allows the passivation layer present on thealuminium surface to be dissolved and then allows an amalgam to beformed with the mercury. This amalgam protects the aluminium part fromthe phenomenon of aluminium passivation by oxygen and catalyses thealumina formation reaction, by the reaction of the aluminium ions in theopen air.

However, the method explained above allows aluminium monoliths to begrown at the very most to a few millimetres in size, provided, inaddition, that the heat released by the reaction is vigorously removed.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to propose a method ofpreparation of a monolithic hydrated alumina, the said method allowingeffective growth of alumina from a part made of aluminium or analuminium alloy.

Other objects of the present invention are also to propose methods ofpreparation of monolithic, amorphous or crystalline, anhydrous aluminas,monolithic aluminates and monolithic composites from hydrated aluminasproduced according to the invention.

To do this, the invention relates to a method of preparation of amonolithic hydrated alumina, the said method comprising, in succession,the following steps:

-   -   a) abrading of a surface of a part made of aluminium or an        aluminium alloy;    -   b) covering of the said surface with a mercury amalgam        comprising at least one noble metal; and    -   c) exposure of the said covered surface obtained at b) to a wet        oxidizing atmosphere.

According to the invention, it should be pointed out that, in what wasmentioned above and follows later, the term “surface” is understood tomean “at least a portion” of the said part.

It should be noted, according to the invention, that the abrading stepis intended to remove the passive oxidation layer possibly present onthe surface of the aluminium or aluminium alloy part and may be carriedout, for example, by cleaning the surface using a solvent, so as toremove the organic materials possibly deposited on the said surface, itbeing possible for this cleaning operation to be followed by mechanicalabrading of the alumina layer (called the passive oxidation layer), forexample by abrasion with water.

Advantageously, the method allows an ultra porous hydrated aluminamonolith to be obtained that may have, depending on the experimentalconditions, a density of 0.01 to 0.05 g/cm³.

It is also possible with this method, according to the invention, toform monoliths, the cross section and the shape of which correspond tothose of the surface of the part made of aluminium or an aluminiumalloy, which serves as a base for depositing the amalgam, and the heightof the said monolith is controlled solely by the imposed growth time.

According to the method of the invention, it is also possible to controlthe monolithic growth, for example by recovering the product formed whenit has reached a desired height or by leaving the reaction to continueuntil the constituent aluminium or aluminium alloy of the part has beencompletely consumed, something which was not possible in the preparationmethods of the prior art. Thus, with the said method, it is possible toobtain, in the case of complete oxidation of the aluminium, a monolithheight which may reach, for example, close to 1000 times the thicknessof the starting aluminium part.

In addition, the spongy structure of the monolithic hydrated aluminasobtained by the method allows large quantities of fluids to be absorbedand also allows physico-chemical or thermal exchange with gaseousphases, for example the ambient air, to take place.

It is thus possible to recover, within the hydrated monoliths, anhydrousliquids such as oils or various polluting or polluted products. Byexchange with gaseous phases it is possible, for example, to fix, andthus remove from the ambient air, acid or basic vapours or various otherpollutants.

Preferably, the method according to the invention furthermore includes astep of cooling the surface of the aluminium or aluminium alloy part,the said step being carried out simultaneously with the exposure stepc). This is because, to ensure correct growth of the monolith, it ispreferable to limit the temperature of the aluminium or aluminium alloyto maximum values of around 40-50° C., and preferably to temperaturesclose to the ambient temperature. Now, hydration and oxidation reactionsoccurring during the exposure step are highly exothermic and generate agreat deal of heat, which may prevent the method from proceedingcorrectly. Thus, the amalgam-treated surface may advantageously becooled via a heat extraction system, for example by circulation of arefrigerating liquid, the said system being connected directly to thecovered surface obtained at step b). The cooling may also be carried outby fixing the aluminium or aluminium alloy surface to a metal blockcooled by circulation of water coming from a cryostat. However, it ispreferable not to cool down to too low a temperature so as to avoidcondensation of the moisture in the wet oxidizing atmosphere.

According to one particular way of implementing the invention, themethod may furthermore include, so as to improve the monolithic growth,at least one step of regenerating the surface covered with amalgam, thesaid regeneration step consisting in removing the amalgam previouslydeposited and then in redepositing an amalgam, as defined above, and inagain exposing the newly covered surface to a wet oxidizing atmosphere.

For example, the amalgam is removed from the surface by a mechanical orchemical treatment, such as an acid washing treatment, which will removethe amalgam including impurities possibly present on the said surface.

This method of implementation is particularly advantageous when thesurface of the aluminium or aluminium alloy part contains impurities,such as iron, copper or silicon, which impurities may contribute toprematurely impeding the monolithic growth when the content of theseimpurities is greater than a few hundred parts per million in thealuminium or aluminium alloy. The regeneration treatment may consist inremoving the impure species present at the interface between the basesurface and the amalgam by an appropriate treatment, such as acidwashing. Such washing will remove the contaminated amalgam, which wouldno longer ensure correct alumina growth. The removal must be followed bya fresh deposit of amalgam, in order to restart the monolithic growthfrom the reactivated surface.

Preferably, the aluminium surface is a surface having an aluminiumcontent of 99.99 to 99.999% by weight.

Advantageously, the use of an aluminium with such a content, within thecontext of this method, ensures monolithic growth without anyretardation, owing to the minute presence of impurities within thismaterial.

According to the invention, the aluminium surface may be a surface madeof an aluminium alloy containing, for example about 1% magnesium.

The use of such an alloy does not hinder the monolithic growth ofalumina because the magnesium, depending on the operating conditionsapplied, will oxidize without impeding the oxidation of the aluminium.

The mercury amalgam used within the context of the invention is amercury amalgam that contains at least one noble metal.

The term “noble metal” is understood according to the invention to meana metal that does not oxidize, either in air or in water, that is notattacked very easily by acids.

According to the invention, the noble metal included in the amalgam maybe chosen from the group consisting of silver, gold, palladium,platinum, rhodium, iridium, ruthenium and mixtures thereof.

Preferably, the noble metal used in the mercury-based amalgam is silver.The use of such a metal is particularly advantageous as this is arelatively inexpensive noble metal, in particular in its nitrate form,which also gives excellent results within the context of this method.

According to the invention, the silver content of the mercury amalgammay range from 1 to 43 at %, preferably substantially equal to 40 at %,of silver.

The optimum noble metal content of the amalgam in order to obtain agiven monolithic growth rate and a given density of the monolith may bereadily determined by a person skilled in the art.

According to the invention, the step of covering the aluminium oraluminium alloy surface with the amalgam may be envisaged in variouspossible ways.

According to a first possible way, the covering step may be carried outby direct deposition of the amalgam in liquid form on the surface to becovered. This version requires prior preparation of the amalgam beforedeposition. For example, the amalgam is prepared by mixing a definedquantity of solid noble metal into liquid mercury.

According to a second possible way, the covering step may be carried outby depositing a mercury salt and at least one noble metal salt directlyon the surface, the amalgam forming directly on the said surface by anoxidation-reduction reaction. In general, the mercury salt and the saltof the chosen noble metal are in the form of solutions of mercurynitrate and of the nitrate of the chosen noble metal. For example, thesesolutions have a mercury concentration of 0.05 to 0.1 mol/l and a noblemetal, such as silver, concentration of 0.001 to 0.03 mol/l.

According to this second possible way, either the deposition is carriedout in two steps, for example by immersing, in succession, the surfaceto be covered in a mercury salt solution and then in a noble metal saltsolution, or in a single step, for example by immersing the surface tobe covered in a solution containing both salts. It should be noted that,according to these two alternatives, the noble-metal-based mercuryamalgam forms directly on the aluminium or aluminium alloy surface.

According to one particularly advantageous method of implementing theinvention, the oxidizing atmosphere, in which the hydrated alumina ismanufactured according to the invention, is air. Thus, the alumina maybe manufactured, according to this method of implementation, directly inthe open air, without having to use a suitable oxidizing atmosphere.

Preferably, the moisture content of the oxidizing atmosphere ranges from20 to 99.99%. Such a moisture content allows a correct hydrationreaction to take place, in so far as the growth rate of the monolith issubstantially proportional to the moisture content. To benefit from sucha moisture content, the usual relative humidity of temperate climatesmay be very satisfactory. However, in very dry weather, having an openwater-filled container nearby may provide sufficient humidity. If it isdesired to ensure that the growth conditions are strictly controlled, itis possible to regulate the relative humidity, for example using anenvironmental chamber or any similar device.

The temperature at which the exposure step c) may be carried out may besubstantially the ambient temperature. However, of course thistemperature must preferably not be too low, so as to prevent theatmospheric moisture from condensing.

Thus, there is obtained after carrying out the method according to theinvention a monolithic hydrated alumina consisting of one phase, thecomposition of which is close to Al₂O₃.4H₂O. This alumina is ultralight, of low density, which may be between 1×10⁻² and 5×10⁻² g/cm³ witha porosity possibly greater than 99%, and of high specific surface area,which may range from 300 to more than 400 m²/g.

From the structural standpoint, the hydrated alumina obtained accordingto the method of the invention comprises an assembly of nanostructuredentangled fibres with a mean diameter, for example, of about 5nanometres, in such a way as to form a monolith. Apart from theinterstices between the fibres, which form a randomly directed porosity,the alumina may include, according to the invention, another porosityformed by channels having a mean diameter of a few microns, the saidchannels being mutually parallel and oriented along the growth directionof the material.

The object of the present invention is also methods of manufacturingmonolithic amorphous anhydrous aluminas or monolithic crystallineanhydrous aluminas from the hydrated aluminas obtained according to themethod described above.

Thus, it is possible to produce, from a hydrated alumina obtainedaccording to the method described above, a monolithic amorphousanhydrous alumina by heating the said hydrated alumina to a suitabletemperature, i.e. a temperature allowing the initial hydrated alumina todehydrate.

According to the invention, it is also possible to prepare a monolithicalumina crystallized in the δ, γ, θ, κ, κ′ or α alumina form by heatingthe aforementioned hydrated alumina to a suitable temperature. In thiscase, the expression “suitable temperature” is understood to mean atemperature that allows the crystalline phase in question to beobtained. This temperature may be readily determined by a person skilledin the art using techniques such as X-ray diffraction analysis, thesuitable temperature being chosen when the peaks relating to the desiredcrystalline phase appear in the X-ray diffraction diagram.

In general, the heating temperature for obtaining anhydrous aluminas mayfor example be from 200° C. to about 850° C. for an amorphous alumina,from about 850° C. to about 1100° C. for a γ-alumina, from about 1100°C. to about 1200° C. for a θ-alumina, and above 1200° C. for anα-alumina.

It is thus possible to obtain an alumina crystallized in its γ form, thedensity of which may range from 10×10⁻² to 50×10⁻² g/cm³, with a highporosity of around 90% and a specific surface area that may range from100 to more than 150 m²/g.

It is thus possible to obtain an alumina crystallized in its α form, thedensity of which may range from 20×10⁻² to 200×10⁻² g/cm³, with aporosity ranging from 10 to 80%.

However, the density and the porosity of the monolith, for the variousanhydrous aluminas presented above, depend on the temperature and theheating time, these heat treatments being accompanied by a reduction inthe dimensions of the monolith, while still preserving the shape of thestarting product. These parameters may be readily set, by a personskilled in the art, depending on the desired criteria.

It is thus possible to obtain, using these methods, porous monolithicaluminas that are amorphous or are crystallized in various allotropicforms, the said aluminas having a porous structure similar to thestarting product, in so far as the porous structure of the startinghydrated alumina remains during the heat treatments. It should also benoted that the size of the constituent particles of the aluminaincreases with the heating temperature, as does the density, althoughthe specific surface area decreases. These ultra porous crystallinealuminas may be impregnated with a liquid or gaseous phase. It is thuspossible to insert various chemical species into them, such as oxides,polymers, carbon-containing products or divided metals, and to subjectthem to an optional heat treatment, for example to form catalysts,thermal or acoustic insulators, nuclear containment or radioelementtransmutation matrixes, refractory materials having specific properties,infrared-transparent windows, and filtration membranes.

The methods of preparing aluminas of the γ or θ type, called “transitionaluminas”, may include an additional step before the heating step, thesaid additional step being intended to stabilize the said crystallinealuminas, especially heated to temperatures of 1200 to 1430° C., thetemperature range within which the a phase usually forms. Thisstabilization step also allows nanoscale fibres or particles to bemaintained at these temperatures.

Thus, the method of manufacturing crystalline aluminas of theabovementioned type, that is to say of the γ or θ type, may furthermoreinclude, before the heating step, a preliminary step of exposing thehydrated alumina prepared using the method of the invention explainedabove to vapours of at least one oxide precursor at a substantiallyambient temperature.

For example, these oxide precursors may be silica precursors, such astetraethoxysilane or trimethylethoxysilane. It is thus possible toobtain, for these transition aluminas, after heating, a silica contentof 2 to 3% by weight, thus contributing to stabilizing the saidaluminas.

The method of manufacturing aluminas crystallized in the δ, γ, θ, κ, κ′or α form may furthermore include, before the heating step, a step ofexposing the hydrated alumina to acid or basic vapours at asubstantially ambient temperature. For example, the acid vapours arehydrochloric acid vapours.

The basic vapours may be ammonia vapours.

The treatment with acid or basic vapours results in the appearance ofhydrated aluminium chloride when treated with hydrochloric acid oraluminium hydroxide when treated with ammonia. These treatmentscontribute to modifying the temperatures at which various alumina phasesappear. For example, after acid treatment, the κ and κ′ transitionalumina forms appear between 800 and 980° C., and the a phase formsabove 1000° C. For example, after ammonia treatment, the δ form isobtained between 800 and 1000° C., while the θ form is obtained at about1000° C. After heating the ammonia-treated monoliths to temperatures of300 to 800° C., the specific surface area greatly increases, reachingvalues of 400 m²/g, with a microporous surface area of more than 100m²/g. This high microporosity is useful, especially for applications incatalysis.

The subject of the present invention is also a method of preparation ofa monolithic aluminate, prepared from hydrated aluminas, amorphousanhydrous aluminas and/or crystalline anhydrous aluminas that areobtained using one of the methods explained above.

It should be mentioned that, according to the invention, the termaluminate is understood to mean an oxide containing, in its crystallattice, in addition to oxygen, at least two metal elements, includingaluminium.

This method comprises, in succession:

-   -   d) a step of impregnating an alumina with at least one compound        containing one or more metal elements to be introduced into the        alumina, in order to form the aluminate, the said alumina being        produced by one of the methods described above; and    -   e) a step of decomposing the compound introduced at d) by        heating it, followed by a step of forming the aluminate by        heating.

The compound chosen may for example be tetraethoxysilane, in order toform aluminosilicates such as mullite (for example of formula2SiO₂.3Al₂O₃). The compound chosen may be salts of metal elements, inorder to form for example spinels, such as those satisfying the formulaMgAl₂O₄, or to form garnets, such as those satisfying the formulaY₃Al₅O₁₂.

According to the invention, the metal salt may be chosen from the groupconsisting of magnesium, titanium, iron, cobalt, copper, nickel,yttrium, actinide and lanthanide nitrates or chlorides, and mixturesthereof.

The suitable temperatures needed, on the one hand, for the decompositionof the compounds comprising the metal element or metal elements to beintroduced into the alumina and, on the other hand, for the introductionof the said metal element or elements into the lattice of the alumina,so as to form the aluminate, may be readily determined by a personskilled in the art by X-ray diffraction, as already mentioned above.

For example, the step of decomposing the compound comprising the metalelement or elements to be introduced into the alumina in order to formthe aluminate is carried out in air at a temperature substantially equalto 500° C. and the step of forming the aluminate, consisting inintroducing free metal elements into the alumina lattice by thedecomposition of the said compound, may be carried out in air, forexample by heating to a temperature ranging from 700 to 1400° C.

Such aluminates are applicable in many fields, such as thermal oracoustic insulation, catalysis, storage of nuclear waste, preparation ofradioelement transmutation targets, refractory materials having specificproperties, infrared-transparent windows, and membranes.

The subject of the present invention is also the preparation ofcomposite materials based on amorphous and/or crystalline aluminasand/or aluminates produced according to the methods explained above.

Thus, according to the invention, the method of preparation of acomposite material comprising an alumina and/or an aluminate, asproduced above, and at least one other compound and/or elementcomprises, in succession, the following steps:

-   -   f) a step of impregnating the alumina and/or the aluminate with        at least one precursor of the other compounds and/or element(s);        and    -   g) a step of forming the said compound(s) and/or element(s), the        compound(s) and/or element(s) forming with the alumina and/or        the aluminate, after this step, the composite material.

The other compound that may form a composite material with an aluminaand/or an aluminate obtained according to the method of the inventionmay be chosen from a group consisting of ceramics, metals, polymers andmixtures thereof.

An element that may form a composite material with an alumina and/or analuminate obtained using a method of the invention may be elementalcarbon, the said elemental carbon being able to be chosen from the groupconsisting of graphite, pyrolytic carbon, glassy carbon and mixturesthereof.

By way of examples, the compound precursor, when the other compound is aceramic, may be a metal salt chosen from the group consisting of sodiummetatungstate, ammonium metatungstate, zirconium oxychloride, calcium,yttrium, actinide, lanthanide, magnesium, copper, iron, cobalt andnickel nitrates, diammonium titanyl oxalate, titanium and bariumchlorides, and mixtures thereof.

In general, the step of forming the ceramic corresponding to these saltsmay be produced, in air, by heating the said precursors to a temperatureof between 400° C. and 800° C. This heating serves, on the one hand, todecompose the salts, thus releasing the metal elements, and, on theother hand, to oxidize the said metal elements. For example, when thesalt is zirconium oxychloride, the heating step makes it possible toobtain a ceramic of the zirconia ZrO₂ type. This ceramic thus forms withthe alumina and/or the aluminate a composite material.

Such composite materials may be applicable in many fields, such asthermal or acoustic insulation, catalysis, storage of nuclear waste orpreparation of radioelement transmutation targets, refractory materialshaving specific properties, infrared-transparent windows, and membranes.

As examples, the compound precursor, when the other compound is a metal,may be a metal salt chosen from the group consisting of iron, cobalt,copper, nickel, lead, tin, zinc, tungsten and molybdenum nitrates,sodium metatungstate, ammonium metatungstate, salts of noble metals(silver, gold, palladium, platinum, rhodium, iridium, ruthenium), andmixtures thereof.

The step of forming the metal involved in the composition of thecomposite material comprises a step of decomposing the salt or salts,which is carried out in air at a temperature substantially equal to 500°C., by means of which the metal oxide corresponding to the metal elementis obtained after this step, or to a temperature of 800 to 1200° C.,followed by a reduction step by heating the said metal oxide in order toobtain the metal.

When the decomposition step is carried out within the 800 to 1200° C.temperature range, it is possible to form a pure aluminate, by reactionbetween the alumina and the metal oxide formed, especially one with aspinel structure, or a metal oxide/aluminate or alumina/aluminatemixture depending on the inserted alumina/oxide molar ratios.

The reduction step, intended to reduce the metal oxide coming from thesalt or from the aluminate possibly formed, may be envisaged by theaction of a reducing agent chosen from the group consisting of hydrogenand carbon monoxide, at a suitable temperature, preferably ranging from500 to 1200° C.

The advantage provided by such a method is the obtention of compositematerials especially by treatment at lower temperatures than thoseusually required for carrying out the solid-state reaction withconventional powders. This is because, especially owing to the nanoscaledimensions of the constituent grains of the aluminas prepared accordingto the invention, these have an extremely high reactivity.

Such composite materials may be applicable in many fields, such ascatalysis, magnetism, especially for high-frequency applications, and,more generally, any property induced by the dispersion of fine metalparticles in a solid, chemically or thermally stable, material.

As regards the composite materials according to the invention withpolymers, the polymer precursor may be a monomer or a monomer mixture,the step of forming the polymer being a conventional polymerizationstep.

As an example of a monomer, mention may be made of a monomer chosen fromthe group consisting of styrene, aniline, isoprene, ethylene, vinylchloride, butadiene and mixtures thereof.

As regards the composite materials according to the invention withelemental carbon, the precursor of the said carbon may be a hydrocarbon,the step of forming elemental carbon consisting of a thermal crackingstep.

It should be mentioned that, according to the invention, the term“elemental carbon” is understood to mean carbon that may be in the formof graphite, pyrolytic carbon, amorphous carbon, glassy carbon andmixtures thereof.

Such materials may be applicable in many fields, such as thereinforcement and stabilization of plastics, thermal or acousticinsulation, catalysis and applications of nanotubes or fibres, such ascarbon nanotubes or fibres.

The invention will now be described with reference to the followingexamples, these being given by way of illustration but implying nolimitation.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS EXAMPLE 1

This example illustrates the preparation of a hydrated monolithicalumina.

The starting material was a plate of 99.99% pure aluminium 1 mm inthickness.

Firstly, the metal surface was prepared by cleaning it with a solvent(acetone or alcohol) or a detergent, so as to remove the organic matter,and by mechanically abrading the alumina passivation layer by abrasionwith a stream of water. The surface was then rinsed with deionized waterand left covered by a film of water.

Secondly, the surface thus prepared was covered with a liquid mercuryamalgam containing 40 at % silver by rubbing the surface with the liquidamalgam prepared beforehand.

Finally, the covered surface was fixed to a metal block cooled bycirculation of 15° C. water coming from a cryostat. The surface wasexposed to the atmospheric air with a relative humidity of 50% at theambient temperature of 20° C. The oxidation reaction started immediatelyby the instantaneous appearance of a translucent layer visible to theeye after a few seconds. This layer then continued to grow in the formof a monolith perpendicular to the aluminium plate at a rate of 5 mm perhour. After four days of growth, a monolith close to one metre in lengthwas obtained. It had the formula Al₂O₃.4H₂O.

EXAMPLE 2

This example also illustrates the preparation of a hydrated monolithicalumina according to another embodiment.

The starting material was a 99.99% pure aluminium plate 1 mm inthickness.

Firstly, the metal surface was cleaned using a solvent as in Example 1,so as to remove the organic matter, then immersed in an acid or basicsolution, such as a sodium hydroxide or hydrochloric acid solution, fora few minutes in order to remove the passivating alumina layer and thenrinsed with deionized or distilled water, with no subsequent drying.

Secondly, the aluminium surface was dipped in succession, for a fewminutes, into a mercury salt solution and more precisely a mercurynitrate solution having a concentration of between 0.05 and 0.1 mol/l,and then into a silver salt solution, more precisely a silver nitratesolution having a concentration of between 0.001 and 0.03 mol/l.

The exposure step was carried out under the same conditions as inExample 1.

This example resulted in a hydrated monolithic alumina of generalformula Al₂O₃.4H₂O.

EXAMPLE 3

This example illustrates the preparation of a hydrated monolithicalumina according to another embodiment.

The same operating method as in Example 2 was followed, except that thecovering step with the liquid mercury amalgam was carried out in asingle step, by dipping the surface to be treated into a solutioncontaining both salts.

This example resulted in a hydrated monolithic alumina of generalformula Al₂O₃.4H₂O.

EXAMPLE 4

This example illustrates the preparation of nanoscale monoliths ofanhydrous amorphous alumina.

In this example, the hydrated alumina prepared in Example 1, 2 or 3 wasused as starting product. The said alumina was heated to a temperatureof 200 to 870° C.

EXAMPLE 5

This example illustrates the preparation of γ-alumina monoliths. In thisexample, the hydrated alumina prepared in Example 1, 2 or 3 was used asstarting product. The said alumina was heated in air to temperatures ofbetween 870° C. and 1100° C. in order to obtain a monolithic aluminacrystallized in the γ form.

EXAMPLE 6

This example illustrates the preparation of θ-alumina monoliths.

In this example, the hydrated alumina prepared in Example 1, 2 or 3 wasused as starting product.

The said alumina was heated in air to temperatures of between 1100° C.and 1200° C. in order to obtain a monolithic alumina crystallized in theθ form.

EXAMPLE 7

This example illustrates the preparation of α-alumina monoliths.

In this example, the hydrated alumina prepared in Example 1, 2, or 3 wasused as starting product.

The said alumina was heated in air to temperatures above 1200° C. inorder to obtain a monolithic alumina crystallized in the a form.

EXAMPLE 8

This example illustrates the preparation of stabilized γ-alumina orθ-alumina monoliths.

In this example, the hydrated alumina prepared in Example 1, 2 or 3 wasused as starting product. This product was brought into contact withtrimethylethoxysilane (TMES) vapour in a closed container or bycontinuously circulating TMES vapour. The duration of the treatment,carried out at a temperature close to the ambient temperature, was from10 to 120 minutes.

After removing the excess TMES by drying in an oven at 100° C., thealuminas were calcined at temperatures of between 870° C. and 1420° C.

The γ phase was obtained by heating between 870 and 1300° C. The θ phasewas obtained by heating between 1300° C. and 1420° C. The monolithicθ-aluminas thus obtained were able to undergo, without being damaged,the usual preparation conditions for catalysts and in particular theimpregnation with aqueous solutions, while maintaining the integrity ofthe monolith. These monoliths contained 3 to 10% silica by weight.

EXAMPLE 9

This example illustrates the preparation of monoliths of aluminates, forexample mullite.

In this example, the hydrated alumina prepared in Example 1, 2 or 3 wasused as starting material. This product was brought into contact withtetraethoxysilane (TEOS) vapour in a closed container, or bycontinuously circulating TEOS vapour. The duration of the treatment,carried out at a temperature close to the ambient temperature, wasbetween 1 and 6 days.

After removing the excess TEOS by drying in an oven at 100° C., themonoliths were calcined at temperatures of between 980 and 1400° C. inorder to obtain the appropriate aluminate.

EXAMPLE 10

This example illustrates the preparation of aluminate monoliths, forexample a monolith of magnesium aluminate with a spinel structure.

In this example, an alumina prepared in Example 6, 7 or 8 was used asstarting material. The starting material was impregnated with an aqueousmagnesium nitrate solution by immersing the said material in the saidsolution or by infiltrating the said solution into the said material. Bycontrolling the concentration of the salt solution and of the degree ofimpregnation it was possible to obtain the desired stoichiometry forforming the compound MgAl₂O₄.

The alumina thus impregnated was then heated to a temperature of 700 to1000° C. so as to decompose the metal salt and form, by insertion of theMg elements into the alumina lattice, the compound of spinel structure.

EXAMPLE 11

This example illustrates the preparation of monoliths of an alumina oraluminate composite material with a ceramic compound, such as forexample yttrium-stabilized zirconium oxide.

In this example, an alumina or an aluminate prepared in Example 6, 7, 8,9 or 10 was used as starting product. The starting material wasimpregnated with an aqueous zirconium oxychloride/yttrium chloridesolution by immersing the said material in the said solution or byinfiltrating the said solution into the said material. By controllingthe concentration of the salt solution and degree of impregnation it waspossible to obtain the desired oxide composition. This may vary fromless than 1% to more than 80% by weight.

The alumina or aluminate thus impregnated was then heated to atemperature of between 700 and 1300° C. so as to decompose the metalsalt and form the stabilized zirconia.

EXAMPLE 12

This example illustrates the preparation of a composite materialconsisting of an alumina or aluminate (optionally containing anotheroxide) with a metal, for example nickel.

In this example, an alumina or aluminate prepared in Example 6, 7, 8, 9or 10 was used as base material.

The alumina or aluminate was impregnated with a nickel nitrate solutionby immersing the said alumina or aluminate (optionally containinganother oxide) in said solution or by infiltrating the said solutioninto the alumina or aluminate or into the composite with another oxide.

By controlling the concentration of the salt solution and the degree ofimpregnation it was possible to obtain the desired stoichiometry. Thismay vary from less than 1% to more than 80% by weight of metal containedin the composite.

The alumina thus impregnated was then heated to a temperature of 400 to500° C., so as to decompose the metal salt, and then reduced under H₂ orCO at a temperature of 500 to 800° C.

EXAMPLE 13

This example illustrates the preparation of a composite materialconsisting of an alumina or aluminate (optionally containing anotheroxide and/or a metal) with a polymer, such as polystyrene.

In this example, an alumina or aluminate according to Example 6, 7, 8, 9or 10 was used as base material. The base material was impregnated withliquid styrene. This was prepared by washing with a sodium hydroxidesolution, rinsing, drying and then adding benzyl peroxide. The materialwas polymerized in an oven at 80° C. for more than 24 hours.

EXAMPLE 14

This example illustrates the preparation of a composite materialconsisting of an alumina or aluminate (optionally containing anotheroxide and/or a metal) with a carbon product, for example, carbonnanotubes.

In this example, an alumina or aluminate or a composite prepared inExamples 6, 7, 8, 9, 10, 11 or 12 was used as base material. The basematerial was treated with a gas, such as acetylene, ethylene, propyleneor methane, which, by cracking it, for example by heating between 500and 1200° C., gave carbon nanotubes.

BIBLIOGRAPHIC REFERENCES

-   [1]: Kolloid Z., 2, 1908, 11.

[2]: J. Markel et al. in Journal of Non-Crystalline Solids, 180 (1994),32.

1-40. (canceled)
 41. Method of preparation of a monolithic hydrated alumina, the said method comprising, in succession, the following steps: a) abrading of a surface of a part made of aluminium or an aluminium alloy; b) covering of the said surface with a mercury amalgam comprising at least one noble metal; and c) exposure of the said covered surface obtained at b) to a wet oxidizing atmosphere.
 42. Method of preparation according to claim 41, which furthermore includes a step of cooling the said surface, the said step being carried out simultaneously with the exposure step c).
 43. Method of preparation according to claim 42, in which the cooling step is carried out by means of a heat extraction system connected directly to the surface obtained at b).
 44. Method of preparation according to claim 41, which furthermore includes at least one step of regenerating the surface covered with amalgam, the said regeneration step consisting in removing the amalgam previously deposited and then in redepositing an amalgam, as defined in claim 41, and in again exposing the newly covered surface to a wet oxidizing atmosphere.
 45. Method of preparation according to claim 41, wherein the aluminium surface is a surface having an aluminium content of 99.99 to 99.999% by weight.
 46. Method of preparation according to claim 41, in which the noble metal included in the amalgam is chosen from the group consisting of silver, gold, palladium, platinum, rhodium, iridium, ruthenium and mixtures thereof.
 47. Method of preparation according to claim 46, in which the noble metal is silver.
 48. Method of preparation according to claim 47, wherein the mercury amalgam has a silver content ranging from 1 to 43 at % silver.
 49. Method of preparation according to claim 41, in which the covering step b) is carried out by direct deposition of the amalgam in liquid form on the surface to be covered.
 50. Method of preparation according to claim 41, in which the covering step is carried out by depositing a mercury salt and at least one noble metal salt directly on the surface, the amalgam forming directly on the said surface.
 51. Method of preparation according to claim 41, in which the oxidizing atmosphere is air.
 52. Method of preparation according to claim 41, in which the wet oxidizing atmosphere is such that it has a relative humidity ranging from 20% to 99.99%.
 53. Method of preparation according to claim 41, in which the exposure step c) is carried out substantially at ambient temperature.
 54. Method of preparation of a monolithic amorphous anhydrous alumina, which includes a step of heating the hydrated alumina prepared by a method according to claim 41 to an appropriate temperature.
 55. Method of preparation of a monolithic alumina crystallized in the δ, γ, θ, κ, κ′, or α form, which includes a step of heating the hydrated alumina prepared by a method according to claim 41 to an appropriate temperature.
 56. Method of preparation according to claim 55, which includes, when the alumina is of the γ or θ type, before the heating step, a step of exposing the hydrated alumina prepared by a method according to claim 41 to the vapour of at least one oxide precursor at a substantially ambient temperature.
 57. Method of preparation according to claim 56, in which the, or at least one, oxide precursor is a silica precursor.
 58. Method of preparation according to claim 57, in which the, or at least one, silica precursor is chosen from the group consisting of tetraethoxysilane and trimethylethoxysilane.
 59. Method of preparation according to claim 55, which further includes, when the alumina is of the δ, γ, θ, κ, κ′, or α type, before the heating step, a step of exposing the hydrated alumina prepared by a method according to claim 41 to the vapour of an acid or base at a substantially ambient temperature.
 60. Method of preparation according to claim 59, in which the acid vapour is hydrochloric acid vapour.
 61. Method of preparation according to claim 59, in which the base vapour is ammonia vapour.
 62. Method of preparation of a monolithic aluminate, which comprises in succession: d) a step of impregnating an alumina with at least one compound containing one or more metal elements to be introduced into the said alumina, in order to form the aluminate, the said alumina being produced by a method according to claim 41; and e) a step of decomposing the said compound introduced at d) by heating it, followed by a step of forming the aluminate by heating.
 63. Method of preparation according to claim 62, in which the compound comprising the metal element or elements to be introduced is tetraethoxysilane.
 64. Method of preparation according to claim 62, in which the compound comprising the metal element or elements to be introduced is a metal salt chosen from the group consisting of magnesium, titanium, iron, cobalt, copper, nickel, yttrium, actinide and lanthanide nitrates or chlorides, and mixtures thereof.
 65. Method of preparation according to claim 62, in which the step of decomposing the compound chosen is carried out in air by heating to a temperature substantially equal to 500° C.
 66. Method of preparation according to claim 62, in which the step of forming the aluminate is carried out in air by heating to a temperature ranging from 700 to 1400° C.
 67. Method of preparation of a composite material comprising an alumina and/or an aluminate and at least one other compound and/or element, the said alumina being obtained by a method comprising, in succession, the following steps: a) abrading of a surface of a part made of aluminium or an aluminium alloy; b) covering of the said surface with a mercury amalgam comprising at least one noble metal; and c) exposure of the said covered surface obtained at b) to a wet oxidizing atmosphere and the said aluminate being obtained by a method which comprises in succession: d) a step of impregnating an alumina with at least one compound containing one or more metal elements to be introduced into the said alumina, in order to form the aluminate, the said alumina being produced as above; and e) a step of decomposing the said compound introduced at d) by heating it, followed by a step of forming the aluminate by heating, the said method comprising in succession, the following steps: f) a step of impregnating the alumina and/or the aluminate with at least one precursor of the said other compound(s) and/or element(s); and g) a step of forming the said compound(s) and/or element(s), the compound(s) and/or element(s) forming with the alumina and/or the aluminate, after this step, the composite material.
 68. Method of preparation according to claim 67, in which the other compound is chosen from a group consisting of ceramics, metals, polymers and mixtures thereof.
 69. Method of preparation according to claim 67, in which the element is elemental carbon.
 70. Method of preparation according to claim 69, in which the elemental carbon is chosen from the group consisting of graphite, pyrolytic carbon, glassy carbon and mixtures thereof.
 71. Method of preparation according to claim 68, in which, when the other compound is a ceramic, the precursor of this compound is a metal salt chosen from the group consisting of sodium metatungstate, ammonium metatungstate, zirconium oxychloride, calcium, yttrium, actinide, lanthanide, magnesium, copper, iron, cobalt and nickel nitrates, diammonium titanyl oxalate, titanium and barium chlorides, and mixtures thereof.
 72. Method of preparation according to claim 68, in which the step for forming the ceramic is produced, in air, by heating the said precursors to a temperature of between 400° C. and 800° C.
 73. Method of preparation according to claim 68, in which, when the other compound is a metal, the precursor of this compound is a metal salt chosen from the group consisting of iron, cobalt, copper, nickel, lead, tin, zinc, tungsten and molybdenum nitrates, sodium metatungstate, ammonium metatungstate, salts of noble metals (silver, gold, palladium, platinum, rhodium, iridium, ruthenium), and mixtures thereof.
 74. Method of preparation according to claim 73, in which the step for forming the metal comprises a step of decomposing the metal salt or salts, which is carried out in air at a temperature substantially equal to 500° C. or at a temperature of 800 to 1200° C., by means of which a metal oxide is obtained after this step, followed by a reduction step, by heating the said metal oxide in order to obtain the metal.
 75. Method of preparation according to claim 74, in which the reduction step is carried out by the action of a reducing agent chosen from the group consisting of hydrogen and carbon monoxide at a suitable temperature.
 76. Method of preparation according to claim 68, in which, when the other compound is a polymer, the precursor of this compound is a monomer or a monomer mixture.
 77. Method of preparation according to claim 76, in which the monomer is chosen from the group consisting of styrene, aniline, isoprene, ethylene, vinyl chloride, butadiene and mixtures thereof.
 78. Method of preparation according to claim 76, in which the step for forming the polymer consists of a polymerization step.
 79. Method of preparation according to claim 69, in which, when the element is elemental carbon, the precursor of this element is a hydrocarbon.
 80. Method of preparation according to claim 79, in which the step for forming the elemental carbon consists of a thermal cracking step.
 81. Method of preparation of a monolithic aluminate, which comprises in succession: d) a step of impregnating an alumina with at least one compound containing one or more metal elements to be introduced into the said alumina, in order to form the aluminate, the said alumina being produced by a method according to claim 54; and e) a step of decomposing the said compound introduced at d) by heating it, followed by a step of forming the aluminate by heating.
 82. Method of preparation of a monolithic aluminate, which comprises in succession: d) a step of impregnating an alumina with at least one compound containing one or more metal elements to be introduced into the said alumina, in order to form the aluminate, the said alumina being produced by a method according to claim 55; and e) a step of decomposing the said compound introduced at d) by heating it, followed by a step of forming the aluminate by heating.
 83. Method of preparation according to claim 67, wherein the alumina is obtained by a method which includes a step of heating the hydrated alumina to an appropriate temperature.
 84. Method of preparation according to claim 67 wherein the alumina prepared is crystallized in the δ, γ, θ, κ, κ′, or α form, and wherein the alumina is obtained by a method which includes a step of heating the hydrated alumina to an appropriate temperature.
 85. Method of preparation according to claim 71, in which the step for forming the ceramic is produced, in air, by heating the said precursors to a temperature of between 400° C. and 800° C.
 86. Method of preparation according to claim 77, in which the step for forming the polymer consists of a polymerization step.
 87. Method of preparation according to claim 70, in which, when the element is elemental carbon, the precursor of this element is a hydrocarbon.
 88. Method of preparation according to claim 47 wherein the mercury amalgam has a silver content substantially equal to 40 at % silver.
 89. Method of preparation according to claim 74, in which the reduction step is carried out by the action of a reducing agent chosen from the group consisting of hydrogen and carbon monoxide at a temperature ranging from 500 to 1200° C. 